There are many instances where it is necessary or desirable to provide a road or walkway across a waterway to allow vehicle and pedestrian access but which also requires that the bridge be movable to allow boat traffic through the waterway. Existing movable bridges including bridges that are raised upward, either by raising the entire span of the bridge vertically by hydraulics or counterweights to allow boat traffic to pass below the bridge or splitting the bridge in its midsection (bascule bridge) and raising the outer end of each piece of the bridge span to allow boat traffic to pass underneath through the waterway. Other examples of movable bridges include those that rotate on a platform or central piece to allow the bridge to swing out of at least part of the waterway. These bridges are permanent structures and require extensive construction and high cost to install, maintain and operate them. Disadvantages of such bridges are that the extent of bridge movement may be limited so that the height or size of boat traffic may be limited, and they are often extremely expensive to build. Further, many of these bridges still have some bridge structure over the waterway and pose a potential danger to larger ships that may pass through.
Less expensive, temporary bridges may be used for less travelled waterways. One temporary bridge is described in U.S. Pat. No. 3,499,179. This bridge has a plurality of connected sections which are quickly and easily connected together to form the bridge. The bridge may be disassembled and has carrying handles for the plurality of sections. The sections may also be connected to form a raft. It may have pontoons or floats to increase its buoyancy. Further, outboard motors may be attached to propel the raft. However, this bridge/raft is a temporary structure and not suitable as a permanent bridge attached to land which allows vehicle and pedestrian traffic on a long term basis. Further, although the raft may have motors to propel it, the propellers are not meant to move the bridge in and out of position across the waterway, alternating the passage of vehicle traffic over the raft and the passage of boat traffic through the waterway. The raft has no convenient means to propel it between its two positions and would require extensive maneuvering and time to do so.
U.S. Pat. No. 5,263,217 describes a swing bridge for spanning waterways. The bridge is permanently attached at one end to land and removable attached at its other end when spanning the waterway. At its permanently attached end, it is connected by a hinge pivot. The hinge pivot allows the bridge to pivot between its closed position spanning the waterway and an open position where boat traffic can pass through the waterway. The bridge also includes a propeller or jet-type motor which acts transversely to the bridge to advance it between its open and closed positions. The end 21 of this bridge rests in a seat 13. When the bridge is to be moved to an open position to allow boat traffic to pass through the waterway, the length of the bridge is lifted to raise the end 21 out of its seat 13. This requires extensive adjustment of the hollow floating bodies and trim of the bridge. The pivot mechanism for this bridge uses a plurality of wheels 16 on shafts. The idle wheels rest on a base 11 and provide support for the end of the bridge. The pivot system restricts the “pitch” of the bridge. If the bridge pitches during movement, this movement may cause a breakdown of the pivot system. Further, the idle wheels and shafts are subject to considerable forces during pivoting of the bridge and may not be sufficient to withstand these forces in repeated bridge movement. This bridge system could not be used to span large waterways since it could not withstand the stresses involved during the vertical pivot, i.e. when the length of the bridge is lifted from its seat 13, and the structure would fracture or break during the raising of the bridge. It appears that this bridge could only be used to span distances of no more than 100 feet. A more robust bridge system is desirable.